An x-ray generator mainly comprises a vacuum chamber including at one of its ends a cathode and at the other end an anode. An electron beam issued from the cathode is accelerated in the vacuum chamber under the action of an electric field. The accelerator field may be created either by an RF electromagnetic wave, or by a static field created between two electrodes raised to very high electrical voltages (typically from 10 kV to 500 kV). The highly accelerated electron beam is made to strike a target generally consisting of a refractory material of high atomic number or indeed having characteristic fluorescence lines. The interaction between the energetic electrons and the target allows electromagnetic radiation in the x-ray range to be emitted mainly by the bremsstrahlung effect and by x-ray fluorescence. Only a few percents of the energy of the electrons are used to produce the x-rays. The rest of the energy, i.e. about 96 to 99% is dissipated in the form of heat mainly in the target. Under the electron flux, the temperature of the target may approach its melting point, the target generally being made of tungsten (3400° C.). The heat produced must be removed effectively because this effect is one of the main factors limiting the brilliance of x-ray sources. The x-rays then propagate out of the vacuum chamber by passing through a window that is transparent to the x-rays and placed on the path of the x-ray beam.
There are two families of tubes, tubes operating in reflection and those operating in transmission. In a reflection tube, the target is present in the interior of the tube. The flux of x-rays propagates towards the exterior by passing through a window that is almost transparent to the x-rays (for example made of beryllium), which window is located in the chamber of the tube. The target and the transmission window are separate. In tubes operating in transmission, the target, in this case consisting of a thin film, is juxtaposed with the transparent window. The window referred to as the transmission window then plays an important role in the removal of the heat generated in the target. In addition to having a high transparency to x-rays, the transmission window must have a maximal thermal conductivity.
Independently of the positioning of the transmission window, the flux of x-rays may be varied over time, in intensity and in energy in order to adjust imaging parameters such as contrast, penetration power or spatial resolution. The flux of x-rays is in this case varied intentionally. Specifically, an operator may modify the current and/or the acceleration voltage of the electron beam. The flux of x-rays may also vary unintentionally, especially following a fluctuation in the high-voltage supply or in the electron source. In order to use the x-ray generator tube optimally, a direct measurement of the flux of x-rays is required. In radiotherapy, for example, this makes it possible to know in real-time the radiation dose rate delivered by the tube.
Currently, transmission windows are mainly made of beryllium. Beryllium is a metal possessing a good transparency to x-rays and a thermal conductivity of about 200 W/(m·K) at room temperature which allows it to dissipate the heat given off by the x-ray tube in operation. In contrast, beryllium is an extremely carcinogenic material capable of causing serious illnesses if it is inhaled. Mention may be made by way of example of berylliosis. Moreover, since beryllium is a metal, it cannot be used in detection of x-ray flux.
With a thermal conductivity comprised between a few hundred W/(m·K) for synthetic polycrystalline diamond and 2000 W/(m·K) for single-crystal diamond, diamond possesses a thermal conductivity up to 10 times higher than that of beryllium and effectively conducts heat. Diamond of atomic number Z=6 has a transparency to x-rays neighboring that of beryllium (Z=4) and it may be implemented in the form of a UHV-tight membrane. Single-crystal diamond possesses an excellent thermal conductivity and very good x-ray detection properties. In contrast, it is not possible to synthesize an area thereof larger than a few millimeters squared. Polycrystalline diamond may easily be synthesized over larger areas. Furthermore, it possesses a thermal conductivity of 500 to 1500 W/(m·K). The thermal conductivity of polycrystalline diamond depends on a number of factors. Mention may be made, for example, of the density of grain boundaries and the grain size of the crystal structure of the diamond. Moreover, polycrystalline diamond possesses x-ray detection properties that also depend on the quality of the crystal structure of the diamond.
X-ray generators including transmission windows made of polycrystalline diamond have been around for a short while. Replacing transmission windows made of beryllium with transmission windows made of diamond makes it possible to work with higher power densities. Specifically, the high thermal conductivity of diamond makes it possible to better dissipate the heat produced by the electron spot impacting the transmission window. A better dissipation of the heat is particularly advantageous for x-ray tubes that operate in transmission, i.e. with a juxtaposition of the target function and the window function in the same structure. In the prior art, only the high thermal conductivity of diamond is exploited, in order to achieve higher powers than those of conventional tubes.
Currently, the actual flux of x-rays is measured indirectly by associating, by calibration, the x-ray dose with the measurement of the acceleration voltage of the electron beam and of the current of the x-ray tube. However, leakage currents in the x-ray generator, microbreakdowns or indeed energy filtration and absorption effects of the structure of the tube are sources of error because they are integrated into the determination of the flux whereas they do not generate a useful flux of x-rays.
To mitigate this difficulty, a flux of x-rays emitted by an x-ray generator tube may be detected using a sensor, also referred to as a dosimeter. The flux sensor measures the incident flux of x-rays. The flux sensor is placed a few centimeters to a few meters from the tube in the direction followed by the x-ray beam.